Structures of ammonium bromide under high pressure were investigated through ab initio evolutionary algorithm and total-energy calculations based on density functional theory. Static enthalpy calculations indicate that the low-pressure phase V(space group P4/nmm) transforms into a monoclinic P21/m structure at 71 GPa and then an orthorhombic structure Cmma at 130 GPa, which is found to be energetically stable up to 264 GPa. Mechanism of phonon softening at the P4/nmm P21/m transformation is discussed. Ab initio calculations show that the band overlap in the molecular Cmma phase, which causes the pressure-induced insulator-to-metal transition, occurs at about 240 GPa. Enthalpy calculations show that Cmma NH4 Br becomes unstable and dissociates into NH3 and HBr above 264 GPa. Structures of ammonium bromide under high pressure were investigated through ab initio evolutionary algorithm and total-energy calculations based on density functional theory. Static enthalpy analyzes indicate that the low-pressure phase V (space group P4 / nmm) transforms into a monoclinic P21 / m structure at 71 GPa and then an orthorhombic structure Cmma at 130 GPa, which is found to be energetically stable up to 264 GPa. Mechanism of phonon softening at the P4 / nmm P21 / m transformation is discussed. Ab initio calculations show that the band overlap in the molecular Cmma phase, which causes the pressure-induced insulator-to-metal transition, occurs at about 240 GPa. Enthalpy calculations show that Cmma NH4 Br becomes unstable and dissociates into NH3 and HBr above 264 GPa.